notes on interesting digital ephemera

December 01, 2016

The standard story is the steam engine emerged form tinkers and need rather than from science, but ended up fueling science ... "science owes more to steam engines than steam engines owe to science" That has been a matter of dispute in some circles. There were some earlier investigations that were clearly science of the day and important to early progress... I suspect the real truth includes quite a bit of this early science. David Wootton makes a good case in his book The Invention of Science. (recommended read)

November 18, 2016

It's 1940. The Nazis have taken Copenhagen. They are literally marching through the streets, and physicist Niels Bohr has just hours, maybe minutes, to make two Nobel Prize medals disappear.

These medals are made of 23-karat gold. They are heavy to handle, and being shiny and inscribed, they are noticeable. The Nazis have declared no gold shall leave Germany, but two Nobel laureates, one of Jewish descent, the other an opponent of the National Socialists, have quietly sent their medals to Bohr's Institute of Theoretical Physics, for protection. Their act is probably a capital offense — if the Gestapo can find the evidence.

Inconveniently, that evidence was now sitting in Bohr's building, clearly inscribed "Von Laue" (for Max von Laue, winner of the 1914 Prize for Physics) and "Franck" (for James Franck, the physics winner in 1925) — like two death warrants. Bohr's institute had attracted and protected Jewish scientists for years. The Nazis knew that, and Niels Bohr knew (now that Denmark was suddenly part of the Reich) that he was a target. He had no idea what to do.

October 29, 2016

Since 1825 a tradition of The Royal Institution. Michael Faraday's are legend and are still considered an example of presenting science to the public. The Institution has a website with a good number of videos including those, Christmas and otherwise, associated with the Braggs.

June 30, 2016

At breakfast time I was sitting by the house at Vanavara Trading Post [65 kilometres/40 miles south of the explosion], facing north. [..] I suddenly saw that directly to the north, over Onkoul's Tunguska Road, the sky split in two and fire appeared high and wide over the forest [as Semenov showed, about 50 degrees up—expedition note]. The split in the sky grew larger, and the entire northern side was covered with fire. At that moment I became so hot that I couldn't bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few metres. I lost my senses for a moment, but then my wife ran out and led me to the house. After that such noise came, as if rocks were falling or cannons were firing, the earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it. When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn a part of the iron lock snapped.

May 25, 2016

As he began the slow and painstaking process of lowering the tamper, one of his colleagues, Raemer Schreiber, turned away to focus on other work, expecting that the experiment would be uninteresting until several more moments had passed. But suddenly he heard a sound behind him: Slotin’s screwdriver had slipped, and the tamper had dropped fully over the core. When Schreiber turned around, he saw a flash of blue light and felt a wave of heat on his face. A week later, he wrote a report on the mishap:

The blue flash was clearly visible in the room although it (the room) was well illuminated from the windows and possibly the overhead lights. . . . The total duration of the flash could not have been more than a few tenths of a second. Slotin reacted very quickly in flipping the tamper piece off. The time was about 3:00 p.m.

A guard who was stationed in the room to keep an eye on the precious plutonium had little knowledge of what Slotin was doing. But when the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill. Subsequent calculations put the total number of fission reactions at about three quadrillion—a million times smaller than the first atomic bombs, but still enough to send out a significant burst of radioactivity. This radioactivity excited the electrons in the air, which, as they slipped back into an unexcited state, emitted high-energy photons—the blue flash.

April 23, 2016

General relativity tells us that as you get closer to a mass clocks run a bit more slowly. The effect can be observed on Earth with precise clocks at different distances from the center of the Earth and is pronounced enough that you have to correct for it to get the precise timing signals from GPS satellites right.

It follows that time runs more slowly at the center of a mass like the Earth. Apparently Feynman mentioned that, over the life of the Earth, the center of the Earth would be a day or two younger than its surface. The calculation is quite easy and one might expect it assigned as an undergrad physics problem.

It turns out that when you do a back of the envelop calculation you get about a year and a half. Feynman may have made a simple mistake or perhaps he was incorrectly quoted - apparently he never wrote the result down. I was unaware of his result, but I would have probably accepted it even though I had worked it out for distances from the earth (tall buildings, mountains, orbiting satellites).

The paper paper works out the result at the level of an undergrad physics class.

The young center of the Earth

U.I. Uggerhøj,1 R.E. Mikkelsen,1 and J. Faye2

1Department of Physics and Astronomy, Aarhus University, Denmark 2Department of Media, Cognition and Communication, University of Copenhagen, Denmark (Dated: April 20, 2016)

We treat, as an illustrative example of gravitational time dilation in relativity, the observation that the center of the Earth is younger than the surface by an appreciable amount. Richard Feynman first made this insightful point and presented an estimate of the size of the effect in a talk; a transcription was later published in which the time difference is quoted as ’one or two days’. However, a back- of-the-envelope calculation shows that the result is in fact a few years. In this paper we present this estimate alongside a more elaborate analysis yielding a difference of two and a half years. The aim is to provide a fairly complete solution to the relativity of the ’aging’ of an object due to differences in the gravitational potential. This solution - accessible at the undergraduate level - can be used for educational purposes, as an example in the classroom. Finally, we also briefly discuss why exchanging ’years’ for ’days’ - which in retrospect is a quite simple, but significant, mistake - has been repeated seemingly uncritically, albeit in a few cases only. The pedagogical value of this discussion is to show students that any number or observation, no matter who brought it forward, must be critically examined.